1. Field of the Invention
The present invention relates to a data processing device for obtaining machining information required in machining a three dimensional surface of a workpiece, and in particular, to a data processing device for obtaining such information required in machining such as cutting or grinding and in electrical shaping such as discharge or electrolysis.
2. Description of the Related Art
According to a conventional data processing device for obtaining tool locus information in order to perform three dimensional surface machining, a ball end mill is typically used, in which a tool has a cutting portion for machining the three dimensional surface of a workpiece, and the cutting portion includes a cutting edge which directly performs machining to the workpiece. Alternatively, an electrical discharging electrode for surface machining has a portion corresponding to the cutting edge of the tool. The conventional data processing device for three dimensional surface machining merely concerns a tool 1 shown in FIG. 6 in which a profile of the cutting edge is of circular shape. The profile is given by projecting an outermost circumferential locus of the movement (such as rotation) of the cutting edge with respect to a plane directed in parallel with a central axis 2 of the tool motion. In this case, a tool selection unit of the data processing device for use in three dimensional surface machining selects a tool having a tip portion R whose curvature is smaller than a minimum curvature at the recessed or indented portion of the three dimensional surface of the workpiece to be machined. An operation unit calculates a cutting amount at the machining point or tool offsetting amount as tool locus information. This calculation is made on the premise of a circle having a radius R which is identical with the radius R of the tip portion of the tool regardless of the position of the exact machining point on the peripheral surface of the cutting edge. For example, in case of a pick-feed operation, a shape of the tool is defined by a cross-section provided by a normal vector at the machining point and a tangent vector with respect to a pick-feed direction. Therefore, data processing would be easily made if the data on the machining surface is provided.
However, generally, the three dimensional surface of a workpiece does not provide a uniform recess having uniform curvature. Assuming that the cutting edge of the tool has a circular profile, a tool to be used must have an extremely small diameter so as to avoid overcut, if there is at least one portion A having an extremely small radius of curvature or a recess with maximum curvature on the machining surface 3 of the workpiece as shown in FIG. 7. Accordingly, a remaining surface portion B which has a relatively large radius of curvature, or is relatively flat, is also subjected to machining by that tool having, a small diameter. As a result, machining efficiency may be lowered, and an uncut area would be increased in the pick feed motion of the tool. If a predetermined surface roughness is intended, the machining efficiency is lowered, and if the primary attention is drawn to the machining efficiency, the amount of undercut is increased in the pick feed, thereby degrading the surface machining.